We have constructed simian virus 40 (SV40) recombinants carrying a variety of chromosomal eukaryotic genes. These recombinants, together with a complementing helper virus, have been introduced into cultured monkey cells where they replicate to levels of 100,000 copies per cell. Such experiments allow us to examine regulatory sequences, such as promoters and splicing sites, and also to produce substantial quantities of useful gene products, such as human growth hormone and hepatitis B surface antigen, in cultured cells.